In existing conventional plants for cleaning and bleaching of secondary fibers, such as salvaged old papers and other reused or recycled fiber-containing materials, these materials are collected and segregated in piles according to type (ledger, newspaper, cardboard, etc.). The fiber materials from these piles are blended in a hydropulper, pulped and cleaned, followed when required by a cleaning step carried out by water washing or by flotation. The cleaned and deinked pulp, typically containing from 5 to 10% of the original ink present in the waste paper, is then directed to the bleach plant where it is subjected to one or more bleaching stages, each stage involving distinctly different treating chemicals and process conditions. The initial pulping and washing eliminates most of the contaminants (including ink) present in the fiber, and the subsequent bleaching stages brighten the pulp. The effectiveness of the pulp bleaching is measured and designated by parameters of brightness, contaminant count, and viscosity (a measure of pulp strength).
For the cleaning and bleaching of recycled waste paper and other secondary fiber materials, the most common practices employ chlorine-based chemicals (generally for fine papers) or hydrosulfide (generally for newspaper stock). Various treating sequences are utilized in the bleaching of the recycled pulp to attain desired brightness levels, typically above 76 brightness (GE) and commonly in the range of 78-Over 80% of the fine paper (ledger) bleaching utilizes a CEH, CH, or H sequence. As commonly employed in the art these letter designations respectively stand for:
C=Chlorination with chlorine (Cl.sub.2) PA1 E=Alkali extraction with NaOH PA1 H=Alkaline hypochlorite (NaOCl)
Concern over the negative impact on the environment of chlorine-based bleach plant effluents has accelerated in recent years, particularly since the discovery of the highly toxic chlorinated dioxins and furans in some bleach plant effluents, sludge, and pulp products. Today it is generally accepted that it is critical to reduce the amount of chloro-organics in pulp products and in the plant effluents.
Since formation of chlorinated organics is strongly related to the use and consumption level of molecular chlorine in the chlorination and hypochlorite stages of the conventional bleaching sequence, it is of greater importance to minimize formation of chloro-organics through a cost-effective means to reduce the amount of chlorine utilized in the chlorination and hypochlorite stages, rather than rely on post-treatment technologies such as advanced wastewater treatment systems. Unlike the kraft paper industry (virgin wood processing), the recycled paper industry is only beginning to deal with the issue of chlorinated toxins including the initiation of projects involving the reduction of chlorine and hypochlorite.
Most of the more common treating sequences employed or proposed for bleaching and delignifying of kraft pulp as well as those concerned with bleaching of secondary fibers are chlorine based. While some experimental research has been directed to the use of molecular oxygen in processes for bleaching of secondary fiber (Markham, L. D., et al., TAPPI 1988 Pulping Conference Proceedings, pp 189-196), these attempts generally have failed to produce a pulp with acceptable brightness without also using comparatively large amounts of chlorinated chemicals in the bleaching sequence.
U.S. Pat. No. 4,360,439 discloses a deinking agent composition comprising a salt of a fatty acid, a nonionic surfactant, an anionic surfactant, a sodium carboxycellulose, and an alkaline inorganic salt of the group comprising metasilicate, disilicate, carbonate, borate, and polyphosphate. An improved washing process for deinking printed waste paper is also disclosed.
A process for the bleaching of waste paper containing encapsulated constituents such as inks is disclosed in U.S. Pat. No. 4,381,969 wherein the paper is pulped in the presence of an alkaline solution containing peroxide.
U.S. Pat. No. 4,390,395 describes a process for disintegrating waste paper to form pulp which comprises disintegrating waste paper in the presence of an aqueous phase and a peroxide bleaching agent at a high consistency in the range of 22 to 45%. Surfactants are optionally added during the disintegration step.
A method for the oxygen-alkali treatment of waste paper broke containing wet strength resins is disclosed in U.S. Pat. No. 4,416,727. The oxygen-alkali treatment liberates fiber from the resin, and the fiber can be recycled to the papermaking process to make recycled paper products.
Related German Democratic Republic Patents DD 247,932 Al and DD 247,934 Al disclose a method for improving the strength and optical homogeneity of deinked waste paper pulp by treating the pulp with oxygen and alkali following conventional dispersion and cleaning steps.
An improved method for the deinking of cellulosic materials is disclosed in U.S. Pat. No. 4,935,096 wherein the material is dispersed, pulped, and washed to remove liberated ink. An ionic surfactant is added to any of the dispersion, pulping, and washing steps.
The present invention as disclosed and claimed below offers an improved method for the cleaning and bleaching of secondary pulps which reduces the amount of subsequent bleaching required to produce recycled paper products having satisfactory brightness.